Foams and cellular materials are well established as energy absorbing materials. Styrofoam packaging for computer monitors during shipping, bubble wrap and cardboard are examples. Their energy absorbing capability arises from the deformation (bending, stretching, or buckling) of struts (if open-celled) or membrane walls (if closed-cell) [1]. Closed-cell foams may also rely upon compression of a gas contained within the cell to absorb impact energy.
Although the foamed and cellular materials we are most familiar with for packaging and energy absorption are practically all polymeric, recent developments have led to a variety of metallic counterparts. Metals and alloys are attractive in these applications owing to their much higher stiffness and strength, and the increased energy they can absorb by deforming plastically (e.g., by dislocation glide). Foamed aluminum alloy can now be produced by injecting air (e.g., Metal Foams: A Design Guide by Michael Ashby, Anthony Evans, Norman Fleck, Loma Gibson, John Hutchinson and Haydn Wadley, Butterworths, 2000) or a foaming agent (TiH2) into the liquid melt and solidifying the froth in a steep temperature gradient at the surface of the bath [2], or by a similar, semi-solid process [3]. These low-cost, high-volume processing routes strongly encouraged increased research into their performance, and the relation between (porous) structure and performance.
Recent developments by Sypeck and Wadley [4] have led to the development and demonstration of low-cost cellular metals based on woven wire truss structures and formed metal lattices. It is worthwhile to briefly describe both of these approaches/materials.
Woven wire truss core sandwich panels use a core consisting of lamina of woven wire sheets, which are stacked and bonded using brazing or liquid phase sintering. The lightweight (relative density below 10%) cores are then bonded to metallic face sheets, which may be of the same or a different alloy than the core. Compared to metal foam core panels, such cellular metal beams and panels have been shown to exhibit excellent specific stiffness and strength [5]. The formed metal lattice, or simply truss-core, material is produced by first punching a honeycomb-like array of hexagonal holes from a flat dense sheet of the desired metal or alloy (e.g. 304 stainless steel); specially-designed tooling (consisting of a pair of interpenetrating arrays of pins) is then used to deform the hex sheet into an array of tetrahedral. This is accomplished by pushing every second vertex (i.e., node at which three ligaments within the hex sheet come together) upwards, and at the same time, all other vertices in the opposite direction. The resulting truss-core sheets can then be stacked, either with intermediate sheets (or punched hex sheets), and bonded by brazing, sintering, etc.
The cellular metal structures based on woven-wire and truss-core approaches are attractive as structural materials because of their exceptional specific properties. However, they are perhaps even more promising as candidates for multifunctional materials applications. In addition to bearing forces and moments as an integral structural component, they will simultaneously be used as heat exchangers, filters, catalysts, batteries, energy absorbers, or actuators. Cellular metals are attractive as energy absorbers not only because they can double as structural members with high specific stiffness and strength. But also because plastic deformation of metals and alloys is an efficient energy absorption mechanism, such materials have very low Poisson ratio (they densify while crushing), and the threshold stress for crushing can be accurately controlled by the cellular morphology (size, shape of cells and of the struts and cell walls).
Recent work by Elzey et al. [6] has led to the development of active, shape-morphing structural components based on 2-D and 3-D truss-core structures combined with SMA elements (actuators). This design has been shown to provide the capability for fully reversible, shape-changing structures. Applications might include mission adaptable wings for aircraft, tunable rotors for helicopters and turbine generators, and deployable space structures. The SMA elements used currently are based on roughly equi-atomic NiTi, which can be induced to undergo a phase transformation from its martensite form (monoclinic crystal structure) to austenite (cubic crystal structure) either by increasing the temperature to above the austenite finish temperature (Af), or by applying stress at temperatures below the temperature at which the austenite phase is stable. Deformations of up to 8% strain can be absorbed at low temperature (e.g.
20) by the formation of the martensite phase, and are completely recoverable upon heating to the Af temperature.
The present invention relates to, among other things, low-cost cellular metals (e.g. truss-core sandwich panel) and active structures to achieve active, cellular metal materials for use as deployable and reusable energy absorbers and self-healing structural members. The present invention provides low-cost precursors such as the truss-core lattice in combination with active (SMA) elements. The active elements will provide for energy absorption by inelastic deformation (like a conventional metal or alloy), but fully recoverable (like a polymer foam).